The present invention relates to a wind energy system and a method of operating and, in particular, controlling a wind energy system. In particular, the present invention relates to a method of determining the correct operational condition of a wind energy system.
It is known that the operation of a wind energy system depends on the wind speed. Wind energy systems have predetermined wind speed threshold values for determining the operational condition of the wind energy system. For instance, one threshold value is the cut-in wind speed which is defined as the wind speed where the wind energy system starts to generate power. Another example is the cut-out speed which is defined as the highest wind speed during which the wind energy system may be operated while delivering power. Normally, generating energy is stopped at wind speeds higher than the cut-out speed.
It is known to provide wind energy systems with a wind speed sensor such as an anemometer for measuring the wind speed. The signal from the anemometer is provided as an input signal for the controller. For instance, functions like the cut-in wind speed and cut-out wind speed are determined on the basis of that anemometer signal. In the event that no anemometer signal can be provided, the wind energy system cannot be operated. Wind energy system availability is therefore affected by the availability of the anemometer signal.
Known anemometers are mounted on the nacelle of the wind energy systems in the wake of the rotor. Hence, the anemometer signal is influenced by the rotor. The influence on the air flow depends on the variety of operational and boundary conditions. It is difficult to correct the signal measured for these conditions. As a result, the readings of the anemometer are highly inaccurate. Further, the anemometer instruments are normally not calibrated individually at each wind energy system which further contributes to the overall uncertainty of the wind speed measurement. As a result of these uncertainties, the threshold values for control decisions based on the anemometer positioned on the nacelle have to be chosen rather conservatively which leads to unnecessary down-times during which the wind energy system could actually be operating within its design limits and produce energy.